Séminaire Eva Sintes

Abstract :Microbial biogeography and biodiversity have been studied extensively over the past decades. Most studies focused on the taxonomic characterization based on 16S rRNA gene. However, it has become clear that functional traits rather than specific phylogenetic groups are fundamental in generating the biogeographical patterns of microbial communities due to the selective pressure exerted by the environmental conditions and to the functional redundancy present among different taxa. Different functional genes were characterized over a long latitudinal transect (70ºN to 50ºS) and over different depth layers in the Atlantic to unravel the biogeography of marine microbes and their relationship to environmental variability. Genes related to the main pathways of the nitrogen cycle (ammonia oxidation, nitrate assimilation, and nitrite reduction in Bacteria and Archaea) as well as genes related to the inorganic carbon fixation and sulfur oxidation in Bacteria were quantified by q-PCR and sequenced by Sanger- or 454-pyrosequencing. The abundance and relative contribution of the different genes were related to the environmental conditions and showed a distinct depth distribution from the epi- to the abyssopelagic realm, corresponding to the oceanographic provinces as described by Longhurst. Moreover, the diversity of the different genes showed distinct biogeographic patterns, bipolar for ammonia monooxygenase gene and hemisphere asymmetrically for nitrate reductase. Genes related to inorganic carbon fixation and sulfur oxidation were relatively more abundant in tropical and gyral systems and showed a different phylogenetic affiliation in the oxygen minimum zones as compared to the bathypelagic environments. Taken together, our results indicate the potential for ammonia and sulfur oxidation, inorganic carbon fixation and nitrate and nitrite reduction in deep ocean prokaryotes. Besides, the diversity of the different genes allowed us to identify functional biogeographical patterns for rare and abundant taxa and relate them to the environmental conditions.